Subject: a walk in the woods with an eTrex Venture, including in the rain Newsgroups: sci.geo.satellite-nav Date: Wed, 28 Mar 2001 05:05:42 GMT Having recently acquired my first GPSR(GPS receiver), an eTrex Venture, I wanted to test it's reception in a wooded area. These observations were done in the Vancouver BC area, and are based on the GPSR displays, not any analysis of NEMA data, etc. If anyone wants, I'll post the specifics of the trail I used to do the tests, including notes on the easily reproducable locations of waypoints. -------------------------------------------------- The first test was on March 24, between about 3 to 4:30 PM. It was, and had been, a dry, overcast day. Prior to starting this test, I had driven from a nearby location where I can consitently receive from 10 to 12 satellite signals, including WAAS. I had WAAS enabled at the start of this test. While walking down a trail in the forest, I held the Venture level in my hand. At certain locations, I put the Venture down, to let it get a reading while not moving. Altogether, I recorded 6 waypoints. An attempt to get a 7th failed due to loss of signal(only 2 sats 'visible'). The number of satellites that were being received varied from 2 to 5, but other than at the attempted waypoint mentioned above, the Venture never reported a loss of signal. After marking a waypoint at the end of the trail, I disabled WAAS, and waited for a minute or two to see if the Venture 're-used' the WAAS channels for regular GPS signals, which it did not. I then switched to the Nav page, and while walking the trail in the reverse direction, observed the position indicator and accuracy circle in relation to the tracklog trace. The position indicator was not necessarily 'right on top of' the tracklog trace, but it was always within the radius of the accuracy circle from the trace. I tested this at one point by going down a side trail, and it was clear within about 50 feet that I was on the wrong trail. My conclusion was that with a tracklog, or properly selected waypoints(e.g. at or near branches in a trail), that even at night with a flashlight, you could have a reasonable chance of 'finding your way home' with the help of the GPS. -------------------------------------------------------- The second test was on March 27, between about 3 to 4:30 PM. It was, and had been, a rainy day. Prior to starting this test, I had driven from a nearby location where I can consitently receive from 10 to 12 satellite signals, including WAAS. I had WAAS enabled at the start of this test. While walking down a trail in the forest, I held the Venture level in my hand. At certain locations, I put the Venture down, to let it get a reading while not moving. Altogether, I recorded 7 waypoints. The number of satellites that were being received varied from 0 to 4. I was primarily interested in comparisons to my observations from the prior trail walk, which were: 1) Decreased accuracy. Roughly speaking, the accuracy shown on the Venture on the dry day was 4-10 metres, and in the wet was 9-18 metres. 2) More problems with loss of signal. On the dry day, there was only one time when I didn't get a signal, which was at the second spot where I wanted to record a waypoint. Today, I lost the signal a number of times, and had to backtrack to a prior spot to regain it. 3) I have posted this in another thread, but I noticed what appeared to be an issue with reception being affected by having moisture on the Venture case on top of the antenna. After marking a waypoint at the end of the trail, I disabled WAAS, and waited for a minute or two to see if the Venture 're-used' the WAAS channels for regular GPS signals, which it did not. During the trail walk, with WAAS enabled, the Venture was attempting to use 2 channels for WAAS(POR & AOR-W). At the turnaround point, after disabling WAAS, I noticed that the unit had 3 channels not allocated to a satellite, so it must have 'dropped' a regular GPS sat earlier. I reversed the trail walk, this time observing the satellite page, to see when the 'dropped' channels would be 'assigned' to a satellite, even if that signal couldn't be received in the woods. The Venture actually dropped another channel during the walk back, so by the time I was back at the trail's start, I only had 8 possible signals. I left the unit on, and on my dash, as I drove home, and I checked a few times on the way, as traffic allowed. At some point it added a 9th satellite, but I still had 3 unused channels when parked at home, 26 minutes after turning WAAS off. I then turned the Venture off, and then on again, and it immediately added a 10th satellite. At this point I terminated my testing. My conclusion was that reception in the woods, when wet, is degraded, and that it may make a difference if you keep your GPS antenna area dry.